Pre-compute non-participating reviewers on CodeReview

[andymeneely]

For CodeReview, let's compute the number of non-participating reviews for a given code review. We could do it on the CodeReviewParser, but that script is getting big and unwieldy. Another way would be to put it in rake:analysis - which is not much slower with indexing.

We need:

• New column in the database schema
• Actually compute the number somewhere in either the parser or in rake:analysis
• Verify on the development data